This invention relates to a scanning electron microscope equipment for measuring dimensions of minute patterns, and specifically to a scanning electron microscope equipped with a function of controlling a difference in a measured dimension between scanning electron microscope equipment (hereinafter also referred to as SEM for short) and a difference caused by aging of the scanning electron microscope, a method for measuring a pattern using the same, and an apparatus for correcting a difference between scanning electron microscopes.
In the semiconductor manufacturing process, with miniaturization of patterns, dimension measurement equipment with higher measurement accuracy is sought for. Requirements for measurement accuracy include not only improvement in measurement accuracy of the measurement equipment, standing alone, but also reduction of the difference in a measured dimension among a plurality of SEM's installed in a production line and reduction of a variation in measured dimension caused by aging of the SEM.
As a dimension measuring tool for measuring the width of a fine pattern in the order of a few tens of nanometers, there has conventionally been used the scanning electron microscope for measuring a dimension that can image these patterns at 100,000-200,000 times enlargement magnification (length measuring SEM (Scanning Electron Microscope)) or a CD (Critical Dimension) SEM).
One example of dimensional measurement processing using such scanning electron microscope equipment (hereinafter referred to SEM for short) will be explained using FIG. 5. First, an image 0501 of a measurement target pattern is acquired with the scanning electron microscope equipment, then within a selection area 0502 in the acquired image, secondary electron signal waveforms 0503 of the measurement target pattern are weight-averaged in a longitudinal direction of the pattern, further the averaged result is smoothed to be an image profile (sectional waveform of the image) 0504, and finally a pattern dimension 0505 is calculated as a length between two dimensional measurement points detected in the image profile.
In such scanning electron microscope equipment, as a technique of reducing the difference in a measured dimension among a plurality of equipment and a variation in dimension caused by aging of the equipment, there is being used a method in which    (1) The difference in a measured dimension between SEM's was obtained in advance by measuring the same position once with each of the plurality of SEM's, and    (2) When a pattern dimension is measured by each SEM, the difference in a measured dimension between the SEM's that was obtained in the STEP (1) is added to a value of that SEM, whereby the measured dimension is matched with a measured dimension of other SEM.
Moreover, in addition to this, as a technique of measuring the difference in a measured dimension between SEM's and a variation in a dimension caused by aging of the SEM, a technique of measuring a pattern at the same position for a plurality of times with each SEM and calculating a difference of extrapolated values of the measured values obtained with respective SEM's is proposed in “Measurement Precision of CD-SEM for 65 nm Technology Node,” Hideaki Abe et al., Metrology, Inspection, and Process Control for Microlithography XVIII, Proceedings of SPIE, Vol. 5375, pp. 929.[0008]
In the scanning electron microscope equipment that measures a dimension of a pattern, in the conventional technology based method for evaluating the difference in a measured dimension among the plurality of SEM's and the difference in a measured dimension caused by aging, the same position is measured once with each SEM and the difference in a measured dimension is designated as the difference in a measured dimension between SEM's and the difference in a measured dimension caused by aging. However, since the scanning electron microscope has a characteristic that (A) there is a variation in measured dimension when measuring a dimension repeatedly, and that (B) when the same position is imaged for two or more times, each dimensional measurement yields a variation in the target pattern shape and a change in material characteristics, the measurement results when measuring a pattern at the same position once with each SEM includes the difference in a measured dimension resulting from both a measurement error and a dimensional difference caused by deformation of the measurement target pattern itself as well as the difference in a measured dimension resulting from difference of the SEM.
In addition, in the conventional technology, what is used for evaluation of the difference in a measured dimension among the plurality of SEM's and the difference in a measured dimension caused by aging is a pattern measured dimension obtained from the image profile (sectional waveform of an image). On the other hand, in the dimension measurement equipment, such as scanning electron microscope equipment, that acquires an image by irradiating a beam of a finite size on it and measures a dimension using the image, one may point out as one large factor causing a large difference in a measured dimension between SEM's a difference in the shape of an electron beam between SEM's. This case has a characteristic that the difference in a measured dimension differs depending on a measured pattern shape.
FIG. 6A shows three kinds of dimensional measurement target patterns 0601, 0602, and 0603. FIG. 6B shows their image profiles when being measured by a. SEM A. FIG. 6C shows their image profiles when being measured by a SEM B. FIG. 6D schematically shows a relation between CD value and beam diameter. As shown in these figures, the scanning electron microscope equipment has a characteristic that, when the beam diameter of an electron beam is different from SEM to SEM, dimensional differences 0605, 0606, and 0607 differ from one another depending on their shapes of the dimensional measurement target pattern.
For the reasons given above, in the conventional technology, it is necessary to evaluate the difference in a measured dimension, that is, to calculate a parameter necessary for profile matching and perform matching of the difference in a measured dimension using the calculated parameter.